A spark plug is mounted on an internal-combustion engine, and is used for ignition to the air-fuel mixture in a combustion chamber. Generally, a spark plug includes an insulator having an axial bore, a center electrode inserted in a front end of the axial bore, a terminal electrode inserted in a rear end of the axial bore, and a metallic shell provided in an outer circumference of the insulator. Further, in the axial bore, a resistor used for preventing a radio noise due to an operation of the internal-combustion engine is provided between the center electrode and the terminal electrode (e.g., refer to Japanese Patent No. 2800279).
Generally, a resistor is formed by heat-sealing of a resistor composition that contains mainly a conductive material, such as carbon black, and ceramic particles (e.g., glass powder or the like). In the resistor, the conductive material covers the glass so as to form a plurality of electrical conduction paths for electrical connection between the electrodes. Thus, with forming many electric conduction paths, it is possible to effectively prevent a sharp increase in resistance, even though the electrical conduction path is somewhat damaged by oxidation due to electric load or the like.